Update data_juicer/ops/mapper/generate_challenging_qa_mapper.py

Co-authored-by: gemini-code-assist[bot] <176961590+gemini-code-assist[bot]@users.noreply.github.com>

Author: Bat-Reality

data_juicer/ops/mapper/generate_challenging_qa_mapper.py

@@ -139,7 +139,7 @@ def extract_json(self, text):
     def process_single(self, sample=None, rank=None):
 
         if self.category is None:
-            print("This OP requires processing multiple fields, and you need to specify valid `category`")
+            raise ValueError("This OP requires processing multiple fields, and you need to specify a valid `category`")
 
         model, _ = get_model(self.model_key, rank, self.use_cuda())